When climbing a high mountain you get out of breath easily because why?

the air is depleted up to almost 50 percent! And that's just at abc(at base camp)! You still have 2 more camp sites and then the summit(top of Everest)!

I learned that if you were to take a "magical" helicopter to the top of Everest you would only be able to last 2-16 minutes due to altitude sickness. I say "magical" helicopter cause theres no helicopter that can go that high due to lack of oxygen in the air.